How big is the Universe?

Nope, I think that's a pretty good way of explaining it - I'll just add that there is no distance measure at the singularity, so it is meaningless to talk of how "big" it is, but in an open and flat Big Bang cosmology, the Universe is inifnite in extent for any T>0. Most relativists/cosmologists don't realise this at first, and watching their eureka moment can be quite amusing

It's much much more complicated First off, the Universe, whether finite or infinite, is massively larger than the observable Universe. If there are galaxies throughout the Universe, then the furthest apart could be 10¹⁰⁰ lyrs from each other!! And that would be for a very small Universe...I'm not sure about 125 billion ligt years figure - it should be around 78 billion, unless there has been a recent update. This is how far apart two galaxies are *today*, that we can currently see at either side of the visible edge of the Universe (so as they were 12-odd billion years ago)As MrJack has already mentioned, the speed of light has nothing to do with the Universal expansion - this is simply a case of distances getting bigger. There is no actual motion involved. Think of ants on a ballon skin. The balloon skin expanding will carry the ants further away from each other, without them actually moving. It is there own motion across the skin (by walking) that is restricted by the speed of light.

Very true - in fact, those galaxies that we can see almost certainly represent only a miniscule fraction of the total number of galaxies in existence...

Minkowski space-time is the space-time of Special Relativity. Our Universe is not Minkowskian except over small distances. Same way that the ground is not Euclidean (we live on a spheroid) but appears Euclidean over small enough distances. I think I know what you are saying here, but the Planck Scale is a scale in the same sense as the atomic scale, and refers to lengths on the order of the Planck Length (1.6 x 10^-35) We sometimes use the phrase, *THE* Planck Time, to refer to the earliest moments of the Universe following T=0, but *A* Planck time is simply the geometric length of time, 5.4 x 10^-44secs. No, it could well be infinite in extent spatially at all times T>0 (at T=0, length has no meaning - but that doesn't mean zero-sized!), irrespective of expansion.The Big Bang space-time (with Cosmological Constant) is finite in extent in the negative time direction, infinite in extent in the positive time direction, and finite or infinite in extent in the spatial directions. If finite, it wraps back on itself as there is no edge.

No. I'm saying that at T=0, size is undefined, and at T>0 the Universe may be finite (and tiny for small T) or it might be infinite. In the latter case, our observable Universe starts out infinitely small, but the entire Universe is still infinite in extent!